Judging Einstein

Before most physicists would believe the claims of relativity, they required proof—which would come in the form of a solar eclipse

J. Donald Fernie

This year we celebrate the centenary of Albert Einstein's special
theory of relativity. Indeed, 1905 was the year in which Einstein
first gave notice of his astonishing abilities. He was but 26 and
had just earned his doctorate, but that year he published four
papers on separate topics, each of which marked a major advance in
physics. The first of these, on the photo-electric effect (the
subject of Roald Hoffmann's Marginalium in the previous issue),
would bring him the Nobel Prize, but it was the third, on special
relativity, that made him both famous and controversial. A decade
after this flurry of papers, in 1915, he unveiled the theory of
general relativity, shaking again the foundations of science.

So different was relativity from the prevailing beliefs that most
physicists demanded proof that it could explain phenomena that Isaac
Newton's canon could not. Satisfying such demands was difficult,
because the difference between the two models could only be apparent
under extreme conditions. There seemed little hope that any
terrestrial experiment could decide between them, but Einstein later
identified three astronomical tests. The first was the proper
calculation of the orbit of the planet Mercury—a feat that was
beyond Newtonian physics (see "In Pursuit of Vulcan" in
the September-October 1994 American Scientist). The second
test required the comparison of light emitted from atoms in the Sun
with light from similar atoms on Earth—relativity predicted
that the Sun's light would have a longer wavelength (an example of
the so-called redshift). The third test posited that if relativity
was true, then rays of starlight that passed near the Sun would be
bent compared to the same rays when the Sun was elsewhere in the
sky. In each case, the relativistic effects are caused by gravity
from the Sun's huge mass.

Early attempts to perform these tests did not silence Einstein's
critics, because some observations supported his theory and others
did not. Thus, the general theory of relativity yielded a much
better solution to the Mercury problem than did Newtonian models,
but another prediction of relativity, the redshift of the solar
spectrum, could not be verified. (Eventually, astrophysicists
learned that several other factors complicated the observation of
this phenomenon.) So with one result in favor and another in doubt,
the third test became something of a deciding vote for or against relativity.

Einstein first suggested how this light-bending effect could be
measured in 1911. He predicted that those rays of starlight that
passed closest to the Sun would be deflected by 0.85 arcseconds
(0.00023 degree) because of the Sun's gravitational field. However,
stars that appear next to the Sun are only visible during a total
solar eclipse. To test Einstein's hypothesis, one would have to take
photographs during an eclipse that showed background stars near the
Sun's disk and compare them with photos taken months earlier or
later, when the same stars rose in the night sky. Did stars
appearing on opposite sides of the Sun's disk maintain the same
spacing when the Sun was gone, or not?

This prediction seemed easy to check. Many pictures of solar
eclipses already existed, as did photos of the night sky. Even so,
skepticism about Einstein's theory was so prevalent that few
astronomers rushed to their archives. And when they did examine
previous photographs of solar eclipses, they found that the pictures
were unsuited to proving or disproving Einstein's claim: The
telescopes had been set to track the Sun's motion across the sky,
not the stellar motions, and the slight differences between these
perspectives obscured the small, predicted shifts in star positions.
However, as time went by and other experiments gave equivocal
results, the solar-eclipse experiment represented the best chance to
test the truth of relativity.

Hoping for a Dark Noon

As early as 1912 it seemed possible to capture the necessary
photographs with little fuss. In October of that year, a total solar
eclipse was to run across the northern parts of South America, and
the astronomical observatory of Córdoba in central Argentina
was near enough to mount an expedition. Unhappily, almost all of
South America was under clouds that day.

Another suitable eclipse loomed in August 1914, running northwest to
southeast across eastern Europe. Erwin Freundlich, a young German
astronomer, was determined to test Einstein's theory but encountered
grave difficulty raising money for the trip. The scientific
establishment in Germany was uninterested in paying for it, leading
Einstein himself to offer his own none-too-abundant finances. With
so few options, Freundlich appealed to other countries for
collaborators that would help fund the expedition. He had only one
taker: William Wallace Campbell and a team from the Lick Observatory
in California. Later, the Berlin Academy provided additional support.

The eclipse was due August 21, but the team of Germans and Americans
established a camp near Kiev well before that date to prepare for
the event. Unfortunately, history intervened: On August 1, 1914,
Germany declared war on Russia, and the German astronomers were
taken prisoner. Russian forces expelled the older scientists and
held the younger ones as prisoners of war. The Russians did allow
the Americans to stay for the eclipse, but again the sky was totally
clouded out. Campbell later wrote "I never knew before how
keenly an eclipse astronomer feels his disappointment through
clouds. One wishes that he could come home by the back door and see nobody."

The next year, at the height of the First World War, Einstein
published his general theory of relativity. This timing greatly
complicated the theory's dissemination because German scientific
journals were then unavailable to the English-speaking world. It was
an astronomer from neutral Holland who brought word of the new
theory to Britain. Moreover, Britain was going through a period of
almost hysterical opposition to all things German. Ardently opposed
to this mindless, pervasive hatred, a young British astrophysicist
named Arthur Stanley Eddington stood almost alone. Eddington was not
only a rising star in astronomy but a Quaker—a religious
pacifist. As such, he refused to fight in the war, although he was
willing to risk his life providing aid to civilians caught in the
violence. Because of his beliefs, Eddington lived on the verge of
imprisonment during much of the war and suffered vicious attacks for
his pacifism and efforts to counter his peers' nationalistic
hostility toward German science.

Eddington learned of Einstein's general theory from the Dutch
astronomer Willem de Sitter and was immediately taken with it. He
was almost certainly the first (and, for a while, the only)
English-speaker to understand the theory and appreciate its
significance. Eddington grasped the fact that Einstein's new work
meant that the eclipse experiment was an even more significant test
of relativity—the general theory predicted twice as much
deflection of light rays passing the Sun as did the special theory.
Another suitable eclipse would occur in 1919, and although in 1915
there was no immediate hope for peace, the British Astronomer Royal,
Frank Dyson, began to lay plans (no doubt at Eddington's prompting)
for an expedition to photograph the event. Eddington, of course, was
eager to lead such an expedition but worried that his uncertain
standing with the authorities might cause difficulties for the
project. Then, in a stroke of genius, Dyson wrote a carefully worded
letter to officialdom. In response, the government notified
Eddington that he was lucky so far in having avoided prison, and
that his only hope of remaining that way was to lead Dyson's
expedition, whether Eddington liked it or not! Eddington dutifully
bowed to the hoped-for ultimatum.

Partly Cloudy

Around the same time, an eclipse in the United States in June 1918
was almost entirely obscured by clouds, but Campbell's team did get
some photographs. These poorly exposed plates seemed to indicate no
relativistic effects, much to the delight of Einstein's skeptics,
including Campbell.

The eclipse of May 29, 1919, was to start near the border between
Chile and Peru, then traverse South America, cross the Atlantic
Ocean and arc down through central Africa. No part of the path was
far from the equator, and the desirable, longest-lasting portion was
in the Atlantic, a few hundred miles from the coast of Liberia. The
British planners decided that the tiny island of Principe, nestled
in the crook of Africa's Gulf of Guinea, would be best despite the
poor astronomical viewing from low-lying tropical regions. The
choice of Principe introduced other challenges. One modern travel
agency advises prospective visitors to the island that "It's
best to go between June and September. The rest of the year is muggy
and hot—you'll be swimming in rain and your own sweat."
Just in case Principe was cloudy at the crucial time, the British
sent a second expedition to observe the eclipse from Sobral, in
eastern Brazil.

The main instruments at both sites were existing astrographic
telescopes of 33-centimeter aperture designed specifically for
photographing star positions with high precision. Although these
telescopes were designed to automatically follow the stars, their
temporary emplacement in the field required each telescope to be
immobilized as a clockwork-driven flat mirror tracked across the sky
and fed light to the main lens. As an afterthought, the Brazil
contingent added a small 10-centimeter telescope to its roster. In
the end, it saved the day.

The expeditionaries set out months ahead of the eclipse to allow for
travel difficulties. Although the war officially ended in November
1918, chaos continued for months thereafter. Upon arrival, they had
to evaluate the terrain, choose a site, and set up and test their
equipment. Eddington's group arrived at Principe in late April and,
amid the heat and rain, found themselves under such constant attack
by biting insects that they needed to work under mosquito netting
most of the time. The rain grew worse as May advanced, and the day
of the eclipse began with a tremendous storm. The rain stopped as
the day wore on, but the totality phase of the eclipse would start
at 2:15 p.m. and last only five minutes. Eddington wrote:

About 1.30 when the partial phase was well advanced, we
began to get glimpses of the Sun, at 1.55 we could see the crescent
(through the cloud) almost continuously and large patches of clear
sky appearing. We had to carry out our programme of photographs in
faith. I did not see the eclipse, being too busy changing plates,
except for one glance to make sure it had begun.... We took 16
photographs ... but the cloud has interfered very much with the star
images.

The weather in Brazil was much better—beautifully clear, in
fact. The observers took 19 photos with the astrograph and eight
with the small telescope. But when the photographs were developed,
they found that despite their precautions, the astrograph's pictures
showed, according to Dyson, "a serious change of focus, so
that, while the stars were shown, the definition was spoiled."
Even under ideal conditions, the predicted relativistic displacement
on the photographs was only 1/60 of a millimeter—about a
quarter of the diameter of a star on a sharply exposed image.
Although they could measure such a minute shift, the poor focus made
this task nearly impossible. By contrast, the small telescope's
photographs were clear and sharp, but on a reduced scale.

Weighing the Data

Many months later, back in England, Eddington pondered the
inconsistent results. Einstein's theory predicted a displacement of
1.75 arcseconds, but none of the experiments was in perfect
agreement with the theory. The usable photos from Principe showed an
average difference of 1.61±0.30 arcseconds, the astrograph in
Brazil indicated a deflection of about 0.93 arcseconds (depending on
how one weighted the individual spoiled photos), and the little
10-centimeter telescope gave a result of 1.98±0.12
arcseconds. The smaller device, in addition to yielding the most
precise data, afforded a wider field of view and supported
Einstein's theory of how the displacement should vary with angular
distance from the edge of the Sun. But the validation of relativity
required exact measurements, particularly because physicists had
realized that Newtonian theory alone could predict a stellar
displacement that was half that of Einstein's, or about 0.83 arcseconds.

Eventually, Eddington, after much discussion with Dyson, suggested
an overall measurement of 1.64 arcseconds, which he took to be in
pretty good agreement with Einstein, but he also gave the separate
results from each telescope so others might weight them as they saw
fit. Moreover, Dyson offered to send exact contact copies of the
original photographic glass plates to anyone who wished to make
their own measurements, which should have gone far to refute the
occasional allegation that Eddington had cooked the results.

Ironically, confirmation of Eddington's conclusion (and the theory
of relativity) came from Campbell's team at an eclipse in Australia
in 1922, for which they determined a stellar displacement of
1.72±0.11 arcseconds. Campbell had been open in his belief
that Einstein was wrong, but when his experiment proved exactly the
opposite, good scientist that he was, Campbell immediately admitted
his error and never opposed relativity again.

AcknowledgmentI am indebted to Dr. Jeffrey Crelinsten for granting access to
his unpublished work on this topic and for providing comments on
an earlier version of this article.